Surface roughness measuring device



Feb. l, 1949. J, p ARNDT, JR 2,460,726

SURFACE ROUGHNESS MEASURING DEVICE Filed Jan. l1. 1943 Patented Feb. l,.1949

SURFACE ROUGBNESS MEASRING DEVICE John P. Arndt, Jr., Euclid, Ohio,assigner to The Brush Development Company, Cleveland, Ohio,

a corporation of Ohio Application January 11, 1943, Serial No. 471,967

1 Claim. 1

My invention pertains to the measuring of surface roughness and moreparticularly to a new and improved electrical pickup. y

An object of my invention is to provide a new and improved vpickup for asurface roughness measuring device.

Another object of my invention is to provide a more useful and flexiblepickup for a surface measuring device.

It is another object of my invention to provide a stylus system whichwill produce an ,electrical 'output corresponding to either one or bothof two ranges of Wave length of surface irreeularities.

A further object of my invention is to provide a dual stylus system formaking a pickup selectively responsive to either one or more of severalranges of "wave lengths of surface irregularities.

Other objects and a fuller understanding of my. invention may be had byreferring to the following description and claims, ,taken in conjunction with the drawings, in which:

Figure 1 illustrates schematically and in block diagram form anembodiment of my invention.

Figure 2 illustrates another form of my invention.

In the eld of measuring surface irregularities it has been recognizedthat there are several types of surface irregularities. For example, asurface may have waviness due to machining operations and the waves mayhave a peak to trough distance on the order of thousandths to severalhundreds of an inch; and the surface may also have minute surfaceroughness on the order of millionths of an inch.

In present devices a sharp stylus is caused to traverse a surface themeasure of whose roughness is desired, and the sharp stylus follows asclosely as possible the minute irregularities thereby taking on avertical motion in accordance with the extent of the vertical height ofthe roughness. 'I'his vertical motion can be used in a number ofsystems, such for example: as to bend a piezoelectric crystal, actuate amagnetic pickup, vary the space between condenser plates, or actuate amirror on which is focused a beam of light, or any number of othersystems from which an amplification and an indication of the excursionsof the stylus can be had. When the surface to be measured has severaltypes `of roughness with amplitudes of different orders of magnitude adifficulty arises. This diiliculty lis that the vertical excursion ofthe stylus due to surface waviness may be many times the verticalvexcursion due to the surface roughness.

(Cl. 'i3-105) recording system using a light `beam reflected from anoscillating mirror onto a moving strip of fil-m, this dimculty is greatbecause the reiiected light beam may wander oil of the lm due to thewide oscillation of the mirror resulting from the relatively largevertical excursion of the stylus due to surface waviness. This problemis also present in electrical pickup systems but may not be quite asserious as in the purely mechanical pickup system mentioned above, asthe electrical amplifier may have a low frequency cut-off which reducesthe voltage passed to the recording system due to low frequencyactuation of the stylus. The recording system then would pass with fullamplitude only the alternating signals which had a frequency above thecutoi frequency of the electrical system, and a pen recorder such as isused to make a permanent inked picture of the surface roughness, wouldbe actuated mainly in accordance with the higher frequency, and thesehigher frequencies would correspond to the minute surfaceirregularities. However when the amplitude of the waviness is many timesthe amplitude of the roughness, it is diflicult to provide sulcientlysharp low frequency cut-oi to prevent some record of waviness fromappearing on the chart.

My invention is useful when measuring small amplitude roughness, eventhough there is no waviness, as the opposing effect of the second stylussystem is useful to cancel out low frequency vertical vibrationcomponents of the pickup with respect to the specimen which may occurdue to building vibration.

In Figure 1, I show a drive head I0 which is supported on a shaft IIhaving a rack I2 on'4 one side thereof. The shaft Il is supported by aheavy base (not shown). The drive head has an adjustment wheel I3 which,when turned, operates a ,gear wheel having teeth which mesh with theteeth on the rack I2 to adjust the vertical elevation of the head Il!with respect to the base which supports the shaft II. A pickup arm I4extendsfrom the bottom of the drive head I0 and is reciprocated by amotor (not shown) which is in the drive head. It is desirable that thearm I4 be held by thedrive head I0 against up and down motion so thatthere will be no relative motion between the arm I4 and the head I0except the horizontal reciprocal motion which the head imparts to thearm. At one end of the pickup arm I4 there is shown, in much exaggeratedsize, two piezoelectric styli systems 20 and 2 I Ina 3 The stylus system20 is adapted to establish a voltage in accordance with all surfaceirregularities including the minute surface roughness, and the stylussystem 2| is adapted to establish a voltage in accordance with only thelarger surface irregularities, such as waviness,

'I'he stylus system 2|) comprises a bender multiplate type ofpiezoelectric Rochelle salt crystal4 22 which is connected by a cementor other such adhesive to two spaced raised portions 24, 25

'of the housing 23, a sharp pointed stylus 26 which is shown inengagement with a specimen 21 whose surface is to be profiled, and alever system con.. necting the stylus 26 to the crystal 22. The leversystem comprises a lever arm 28 connected at one end to the stylus 26and at the other end to a pivot comprising torsionally flexible anglepiece 29 which is in turn connected to a portion of the arm I4. A spacer30 isprovided between" the angle piece- 29 and the arm 28, and a screw3| or other such attaching meansis used to connect the arm 28, thespacer 3|) and the angle piece 29 to the arm I4. Or, for an alternativeconstruction, the forces in the lever system will be small enough thatinstead of the screw 3| for connecting various pieces together, anadhesive may be used.

One end of a drive pin 32 is connected to the lever arm 28 at va pointbetween the stylus 26 and the pivot piece 29, and the other end of thepin 32 is connected to an end of the crystal 22.

The stylus system 2| is substantially a duplicate of the system 20, andcomprises a similar crystal unit 35 supported in the housing 23 by tworaised portions 36, 31 to which it is cemented,

a stylus 38 in engagement with the specimen 21- and having a blunt orrounded tip which is large compared tothe sharp tip of the stylus 26,and

to the arm I4. A spacer 4| is provided between the angle piece 48 andthe lever arm 39, and a screw 42 or otherl means is used to connect allof the pieces together.

. When a specimen such as 21A is to be tested it is placed under thestyli 26, 38,' and the drive head I0 is lowered by turning theadjustment wheel I3, thereby lowering the pick-up arm I4,

d of the crystals there will now be -aslight bias of the styli towardthe specimen. Upon starting theymotor in the drive head I0, the styli26, 38 will .be reciprocated across the specimen.

Due to the comparatively large area of the tip of the stylus `38 it willnot follow the lminute roughness but will only be actuated verticallybyther waviness, thereby stressing the piezoelectric crystal 35 inaccordance with the surfacevwaviness, and the crystal will generate avoltage between the output wires 45, 46 which is in accordance with thesurface waviness. vAt the same time, due to its small sharp tip, stylus26v will follow` not only the surface waviness but also the surfaceroughness, thereby stressing th piezoelectric crystal 22 in accordancewith the total Avertical excursions of the stylus 26, and the crystal 22will generate a voltage between the output wires 46, 41 which is inaccordance with surface wavlness and surface roughness. The output fromthe two crystals are connected in opposttion, the wire 46 being commonto both, so that the outputs due to waviness substantially cancelthereby leaving substantially only the output due to roughness. That theoutput due to waviness does not cancel entirely isdue to the fact thatthe two stylido not occupy exactly the same position on the specimen,but the cancellation effected will beenough` to prevent a pen of a penrecorder from moving off of the strip of paper on which it is recordingthe profile of the surface roughness. One method of positioning the twostyli close together is to provide the large blunt stylus with a holeextending through it, and to have the sharp stylus extend through thehole and contact the specimen.` The output between the wires 45, 41which corresponds to surface roughness is amplified at the amplifier 50,and the amplified output is fed through the high pass filter 5|, toanother amplifier 52 (if necessary), and then to a utilizing device 53,such forexample as an R. M. S. meter or a pen recorder. The speed of thetrace of thestyli across the specimen, and the low frequency cut-olf ofthe ampliers and-the lter 5I can be designed so that very little signalcorresponding to waviness or vibration which may pass to the amplifier50 due to lack of balance in the two styli systems is passed to theutilizing device.

When it is desired'to record on the utilizing device 53 an outputcorresponding only to surface waviness, the switch 54 is thrownconnecting terminal 55 to the input of the amplifier 56, which nowreceives a signal from the wires 45, 46 which is in accordance with thevoltage generated by the crystal 35. Due to the'low frequency cut-off ofthe -ampliers 50, 52 and especially due to the' filterSI, it isnecessary tov either increase the speed of the trace of the stylus 38across the l" specimen by turning the speed control knob 56, or bycutting out the filter 5|. Cutting out lter 5| can be done by throwingswitches 51, 58.` Obviously', both cutting out the iilter and increasingthe speed can be done ifit is desired. I provide avscrew 59 in threadedengagement with the 'arm I4 for raising the stylus 26 oi of the surfaceof the specimen 21 to prevent damage to the stylus and to thespecimen'kduring rapid tracing. By rigidly supporting the pickupk arm I4in the drive head` I0 the two stylido not have to support the weight ofthe pickup. and therefore the force pressing against the specimen willbe small, thus avoiding damage to soft and 'delicate materials. Twostops60 and 6| are provided for limiting the extent o which the styli 26, 38can be displaced. '1he\` tops 60, 6I may'be small plastic or metalpieces connected to the sidewall of the arm I4. k

AlthoughV I have described my invention in regard to a piezoelectrictype of pickup, it is to be understood that within the scope of myinvention there are many other types of pickups. suchfor example asmoving coil, condenser, variocoupler, unbalanced magnetic bridge. l

jOne such' type is shownin Figure 2v in which there is a first condenserplate 6I) connected to the arm '62 which together with sharp stylus 26forms the stylus system, indicated generally by the reference character20,` and a second condenser plate 6| connected tothe arm 63. whichtogether with blunt stylus- 38 forms the stylus system indicatedgenerally by the reference character 2|. The two arms 62, 63 areypivoted about the same axis 64, and are spaced far enough apart thatthere will be a small space between the condenser plates B0, 6I. Abattery 65 establishes a charge on the condenser 60, 6l, and there is aresistor 66 in series with the battery. The output circuit from thecondenser 60, 6l is supplied to the amplier 6l,and the output from theamplifier 61 may be fed to a meter cr a recorder.

When the two styli 26, 38 move in accordance with surface waviness orvibration and stylus 26 is not moved in accordance with roughness thespacing between the two condenser plates 60, 6l will not changeappreciably, and therefore there will not be a substantial signaloutput. When the stylus 2B moves in accordance with surface roughnessthere will be a differential motion between the plates 60 and 6l, andconsequently there will be a signal output.

Instead of using the system shown in Figure 2 it is also possible to usethe condenser 60, 6l to modulate a signal, and to utilize the modulatedsignal for obtaining an indication of the surf ace roughness.

I have described my invention with a certain degree of particularitywith regard to Figure 1, and briefly with regard to Figure 2. It is tobe understood, however, that many other systems can be devised whichcome within the scope of my invention.

I claim as my invention:

In a roughness measuring device for use on a specimen having surfacewaviness and surface roughness; an arm; motor means for driving said armacross said specimen; first piezoelectric crystal generator meansmounted on said arm; a rst stylus having a tip small enough to followsurface waviness and too large to follow surface roughness; iirst levermeans connecting said rst stylus to said irst generator means forapplying to said first generator means motions of small amplitude andlarge force compared to the motions and force applied to said rst stylusas said iirst stylus is moved across said specimen thereby establishingan electrical quantity in accordance with the waviness induced verticalexcursions of said rst stylus; second piezoelectric crystal generatormeans similar to said rst generator means mounted on said arm; a secondstylus having a smaller tip able to follow the surface roughness as wellas the surface waviness, said second stylus being located on saidspecimen near said first stylus; second lever means connecting saidsecond stylus to said second generforce compared to the motions andforce applied to said second stylusv as said second stylus is movedacross said specimen thereby establishing an electrical quantity inaccordance with the waviness and roughness induced vertical excursionsof said second stylus, the electrical quantity established in accordancewith waviness induced vertical excursions of said second stylus beingsubstantially equal in magnitude to and substantially in phase with theelectrical quantity established in accordance with the waviness inducedvertical excursions of said rst stylus; indicator circuit meansincluding an indicator; iirst electrical connection means connectingsaid rst generator means to said indicator circuit means for giving onsaid indicator an indication of surface waviness; second electricalconnection means including switch means which upon being closedselectively connect said second generator means to said indicatorcircuit means simultaneously with said first generator means so that theelectrical quantity from said first generator means is substantially 180degrees out of phase with and substantially cancels the portion of theelectrical quantity representingsurface waviness from said secondgenerator means whereby there is given on said indicator an indicationof surface rough? ness; speed control means for controlling the speed atwhich said rst and second stylii are driven across said specimen by saidmotor means whereby when said rst generator means alone is connected tosaid indicator circuit means the speed at which said first stylus isdriven across said specimen may be increased; and lifter means connectedto said arm for lifting said second stylus up off of said specimen andmaintaining said second stylus in said lifted position in order that'itstip will not be damaged while said rst stylus traces the specimen athigh speeds.

JOHN P. ARNDT, Ja.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,976,337 Firestone et al. Oct.'9, 1934 2,016,420 Engst Oct. 8, 1935 2,083,759 Temple June 15, 19372,146,442 Price Feb. 7, 1939 2,171,433 Powers Aug. 29, 1939 2,240,278Abbott Apr. 29, 1941 2,329,084 Reason Sept. 7, 1943 2,335,390 Crist Nov.30, 1943 2,396,394 Shaw Mar. 12, 1946

